Rare earths are utilized in numerous commercial products, such as phosphors for lighting, anodic materials for nickel metal hydride batteries, and permanent magnets for electric motors and generators. The recovery and separation of rare earths from natural sources is complicated by the presence of multiple rare earths in most sources, and the chemical similarities between the rare earths. Rare earths are typically found together in minerals, such as bastnaesite, monazite, and xenotime. Many rare earths are similar in features that are typically used for separation, such as elemental size, density, reduction potential, oxidation potential, melting point, and boiling points. Often, low separation factors are achieved for the separation of individual rare earths.
In conventional industrial rare earth recovery processes, rare earth ore is mined from the ground and crushed into a powder. This powder is passed through a series of flotations that separate out waste from the rare earth minerals to form a rare earth concentrate. The rare earth concentrate is chemically processed to remove impurities (e.g., Sr, Th, U, and Ca). Cerium, the most abundant rare earth, can be isolated in its tetravalent state and recovered through oxidization to CeO2. The remaining rare earths undergo a series of dissolutions and precipitations to form purified rare earth chlorides. Liquid-liquid extractions are typically used to separate the individual rare earths. Often, a series of extractions are first used to separate the rare earths into groups containing 3 or 4 rare earths. Then, each group undergoes additional extractions to isolate individual rare earths. In some instances, the separation of rare earths using conventional industrial methods may use at least 750 stages of different liquid-liquid extractions, each often requiring large volumes of harsh chemicals.
Accordingly, improved methods are needed to separate rare earths.